12 Distributed Erlang

A distributed Erlang system consists of a number of
Erlang runtime systems communicating with each other. Each such
runtime system is called a node. Message passing between
processes at different nodes, as well as links and monitors, are
transparent when pids are used. Registered names, however, are
local to each node. This means the node must be specified as well
when sending messages etc. using registered names.

The distribution mechanism is implemented using TCP/IP sockets.
How to implement an alternative carrier is described in ERTS User's Guide.

A node is an executing Erlang runtime system which has
been given a name, using the command line flag -name
(long names) or -sname (short names).

The format of the node name is an atom name@host where
name is the name given by the user and host is
the full host name if long names are used, or the first part of
the host name if short names are used. node() returns
the name of the node. Example:

The nodes in a distributed Erlang system are loosely connected.
The first time the name of another node is used, for example if
spawn(Node,M,F,A) or net_adm:ping(Node) is called,
a connection attempt to that node will be made.

Connections are by default transitive. If a node A connects to
node B, and node B has a connection to node C, then node A will
also try to connect to node C. This feature can be turned off by
using the command line flag -connect_all false, see
erl(1).

If a node goes down, all connections to that node are removed.
Calling erlang:disconnect(Node) will force disconnection
of a node.

The list of (visible) nodes currently connected to is returned by
nodes().

In a distributed Erlang system, it is sometimes useful to
connect to a node without also connecting to all other nodes.
An example could be some kind of O&M functionality used to
inspect the status of a system without disturbing it. For this
purpose, a hidden node may be used.

A hidden node is a node started with the command line flag
-hidden. Connections between hidden nodes and other nodes
are not transitive, they must be set up explicitly. Also, hidden
nodes does not show up in the list of nodes returned by
nodes(). Instead, nodes(hidden) or
nodes(connected) must be used. This means, for example,
that the hidden node will not be added to the set of nodes that
global is keeping track of.

A C node is a C program written to act as a hidden node
in a distributed Erlang system. The library Erl_Interface
contains functions for this purpose. Refer to the documentation
for Erl_Interface and Interoperability Tutorial for more
information about C nodes.

Authentication determines which nodes are allowed to communicate
with each other. In a network of different Erlang nodes, it is
built into the system at the lowest possible level. Each node has
its own magic cookie, which is an Erlang atom.

When a nodes tries to connect to another node, the magic cookies
are compared. If they do not match, the connected node rejects
the connection.

At start-up, a node has a random atom assigned as its magic
cookie and the cookie of other nodes is assumed to be
nocookie. The first action of the Erlang network
authentication server (auth) is then to read a file named
$HOME/.erlang.cookie. If the file does not exist, it is
created. The UNIX permissions mode of the file is set to octal
400 (read-only by user) and its contents are a random string. An
atom Cookie is created from the contents of the file and
the cookie of the local node is set to this using
erlang:set_cookie(node(), Cookie). This also makes
the local node assume that all other nodes have the same cookie
Cookie.

Thus, groups of users with identical cookie files get Erlang
nodes which can communicate freely and without interference from
the magic cookie system. Users who want run nodes on separate
file systems must make certain that their cookie files are
identical on the different file systems.

For a node Node1 with magic cookie Cookie to be
able to connect to, or accept a connection from, another node
Node2 with a different cookie DiffCookie,
the function erlang:set_cookie(Node2, DiffCookie) must
first be called at Node1. Distributed systems with
multiple user IDs can be handled in this way.

The default when a connection is established between two nodes,
is to immediately connect all other visible nodes as well. This
way, there is always a fully connected network. If there are
nodes with different cookies, this method might be inappropriate
and the command line flag -connect_all false must be set,
see erl(1).

The magic cookie of the local node is retrieved by calling
erlang:get_cookie().